1. Field of the Invention
This invention relates to a semiconductor device, and is suitable for a semiconductor device having photodiodes for receiving light.
2. Description of the Related Art
Photodetectors for optical pickup in optical disc players such as compact disc players, in general, not only have the function of receiving a laser beam reflected by a disc surface and outputting an RF signal, but also have the function of outputting a tracking error signal for making the laser beam to follow the groove on the disc and a focus error signal for focusing the laser beam at the level of the disc surface. A photodetector therefore needs to be configured to have a plurality of divisional portions.
A pattern of such a photodetector is shown in FIG. 1. As shown in FIG. 1, the photodetector has six photodiodes a, b, c, d, e and f. When outputs of the photodiodes a, b, c, d, e and f are A, B, C, D, E and F, respectively, an optical system using this photodetector, as shown in FIG. 2, irradiates three laser beams L1, L2 and L3, and outputs an RF signal by A+B+C+D, a focus error signal by (A+C)-(B+D) and a tracking error signal by E-F.
Since photodetectors are devices for converting a light signal into an electric current signal, outputs of photodetectors prepared as discrete devices are very small current signals, and are weak to noise and exhibit a high output impedance. Accordingly, signal lines from a photodetector to a signal processing IC in the subsequent stage need a low-noise design which, in general, is much complicated. To remove this problem, many efforts are being made in recent years to incorporate a photodetector and a subsequent signal processing IC 10 as shown in FIG. 3 to obtain stable output signals.
To incorporate a photodetector and an IC in a single chip, the photodetector needs to be made in the wafer process of the IC. Therefore, optimum design of photodetectors is difficult, and photodetectors incorporated into IC's are often inferior to those prepared as discrete devices. This problem is discussed below in greater detail.
With the optical system shown in FIG. 2, an RF signal and a focus signal are obtained from the central laser beam L1, and a tracking error signal is obtained from the laser beams L2 and L3 at opposite sides. Therefore, if crosstalk among the photodiodes a, b, c, d, e and f is large, it may cause mixture of the focus error signal into the tracking error signal or other undesirable phenomenon.
As shown in FIG. 4, a discrete photodetector comprises a p.sup.+ -type semiconductor substrate 1 with a high impurity concentration, and n.sup.- -type semiconductor layers 2 and 3 lying on the p.sup.+ -type semiconductor substrate 1, so that the n.sup.- -type semiconductor layer 2 and the p.sup.+ -type semiconductor substrate 1 make a photodiode while the n.sup.- -type semiconductor layer 3 and the p.sup.+ -type semiconductor substrate 1 make another photodiode. Since the n.sup.- -type semiconductor layers 2 and 3 are separated by a p.sup.+ -type isolation region 4 with a high impurity concentration, crosstalk between the photodiodes is small.
In the IC wafer process, however, as shown in FIG. 5, a p-type semiconductor substrate 11 with a low impurity concentration is used to improve characteristics of transistors and other integrated devices, and n.sup.- -type semiconductor layers 12 and 13 are made on the p-type semiconductor substrate 11. As a result, even with a p.sup.+ -type isolation region 14 with a high impurity concentration interposed between the n.sup.- -type semiconductor layers 12 and 13, stray carriers (electron-hole pairs) generated in the p-type semiconductor substrate 11 due to incidence of light have long lifetimes and hence increase the crosstalk between the photodiodes.